text:
Instrument landing: why do all electronic devices need to be shut off?
A few days ago, I took a flight home. Towards the end of the journey, the captain asked us to shut down any electronic device that we had on board, including iPods, CD players, phones and so on, because there was very low visibility, and we were starting an instrument approach.
From what I could understand, an Instrument Landing  System (ILS) allows an aircraft to be centred on the approach path, assisted by radio signals delivered by an ILS (a localiser and a glide slope), without any visual reference. These signals are in the 110 MHz (LOC) and 330 MHz (GS) frequency range.
Given that for the take off, visual landing, and the journey itself, shutting off the devices is no longer required (even though there are radio signals being used for communications and navigation), why this extra precaution on a low-visibility ILS approach? What interference could a CD Player possibly produce with the instrumentation to an extent that it's more dangerous than with the normal take off,en route, and landing phases?
On a side note: as you probably understood, my level of expertise is very low, so forgive any dumb mistake or oversight.

related: http://aviation.stackexchange.com/questions/2021

"CD player"? About 8 years ago I was snickered at in a developing country for using a portable CD player. Most recent flight I was on the stew told everyone switch off transmitting functions on their portable electronics as she could see that there were "a lot" still on (presumably SSIDs or whatever from a wifi/bluetooth scan)

Isn't it a general rule that electronic devices always need to be disabled during the take-off/climb and landing phase? I have so far heard a respective announcement on every single flight I have been on, and some airlines even disable the built-in entertainment systems (screens in the seats in front of passengers) during those phases.

@O.R.Mapper before October 2014 they were demanding to switch them off. Then a new regulation came out and now it is possible to keep them on in "flight mode". The flight company always has the last word regarding this allowance of course, but in the last EasyJet flight I took, it was clearly stated that we could keep our devices on during take-off.

@O.R.Mapper That requirement used to be nearly universal, but it's been dropped in some countries. In the U.S., for instance, the requirement was dropped back in late 2013.

@NicolaMiotto, reirab: That is good news for anyone wishing to take photos during take-off and landing :)

It is just a precaution like not turning devices on and off at the petrol station. If it was a real threat, we would have seen many more accidents by now...

Tell ya what - you get you a pilot's license and instrument cert, go up on a day with 0/0 visibility, switch on a hundred electronic devices, make an instrument approach, and let us know how it goes. Then repeat 10,000 times.

@BobJarvis Or I can just post a question here. 0 costs, 0 risks, less time.

There is simply no truth to any of it.  The idea that electronic devices produce magic "interference" that can cause an airplane to crash has been thoroughly debunked, over and over, at least as far back as the 80s.  But if science isn't convincing, how about sociology: it's common knowledge that terrorists exist that want to cause airplanes to crash.  If it was as simple as leaving a laptop turned on (safely stowed in a bag where no one could see it) why hasn't it already happened, ever, even once, in the 13+ years since 9/11?

@MasonWheeler Ummm... no. I'm both a pilot and an engineer who designs electronic devices for a living and I can assure you that electronic devices do, in fact, create RF interference. There's nothing magic about it. Anything with a current flowing through it emits an e-m field. Ideally, this EMI should not be within the frequency spectrum of the ILS, but electronics don't always work like they're supposed to. See my answer for more information. Also, the chance of a laptop crashing a plane is not 100% (or even 1% or 0.001%,) which makes it rather ineffective for terrorism purposes.

@NicolaMiotto I think Bob Jarvis' comment was directed at copper.hat, not you.

@reirab: So first you say "uh huh, magical interference that can crash a plane is totally real and you're completely wrong," and then you say "this magical interference that's totally real can't actually crash a plane."  So which is it?  I am in no way denying the existence of RF interference; what I'm claiming is that the idea that interference from ordinary electronic devices can somehow magically bring down a plane (which is what has been used as justification for making passengers shut them off during takeoff and landing) is utter nonsense and has been debunked repeatedly since the 80s.

@MasonWheeler And what you're saying is wrong and there's nothing 'magical' about it. Is EMI likely to bring down a plane on any given flight? No, of course not. Is it possible for it to cause erroneous ILS indications? Yes, it is. Could that potentially result in crash? Yes, it could. And the FAA doesn't seem to think it's been 'debunked,' which is why they say that pilots will still request passengers to turn off devices during low-vis instrument approaches, even under the new rules allowing the use of electronics during normal landings.

@MasonWheeler If you have a better source for your claim that it's been 'debunked,' please write an answer citing that source. Also, please note that just because something is possible doesn't mean it will happen every time or most of the time. Even if it happens once in every 1,000,000 times, that would be an unacceptably high risk when you're flying 300 people around at ~150-200 mph on an approach.

Short answer: ILS is rather sensitive to interference and not all electronic devices take much precaution in avoiding the generation of interference. The pilot wants to be sure that the readings he's getting on the localizer and glideslope are accurate, since he can't actually see the runway to verify the final approach path visually.
Longer answer: ILS is actually relatively simplistic as RF signals go. Both the localizer and the glideslope just consist of a pair of AM signals that are broadcast directionally. For both systems, the modulated frequencies are 90 Hz and 150 Hz. For the localizer, 90 Hz is broadcast to the left of the runway centerline and 150 Hz is broadcast to the right of the centerline. When the aircraft is aligned with the runway, it will receive equal strength signals of both frequencies. When it's to the right, it will receive more power from the 150 Hz signal and when it's to the left, it will receive more power from the 90 Hz signal. The glideslope works pretty much the same way, just with the 150 Hz signal transmitted below the glide path and the 90 Hz signal transmitted above it. [1] [2]
Now, suppose some passenger's electronic device is emitting a frequency that it shouldn't be. This is not exactly unheard of among consumer electronic devices. This is more common among devices manufactured in countries with less-strict emissions testing requirements/enforcement, but it's possible due to slight defects in almost any device. [Source: I design RF receivers for a living.] Now suppose that this noise happens to be on the localizer or glideslope frequency for the runway to which the aircraft is flying an approach. Hopefully you're starting to see a problem here... Whatever amplitude changes are happening on this unintended emission are now being received by the AM demodulator of the localizer or glideslope, potentially confusing it and, in low visibility conditions, causing the aircraft to be directed somewhere other than the runway threshold. Needless to say, that's bad.
Boeing recommends the following actions for flight crews flying ILS approaches in order to detect erroneous ILS readings:

Crosschecking altitude and DME distance periodically.
Crosschecking altitude and flight management system (FMS) threshold distance.
Crosschecking altitude and the crossing altitude of the outer marker (or locator, very-high-frequency omnirange [VOR] navigation equipment, or FMS).
Crosschecking radio altitude and barometric altitude.
Crosschecking ground speed and rate of descent.
Questioning air traffic controllers when indications do not appear to be correct.

However, even when following those instructions, slightly erroneous ILS signals may be hard to detect until too late. If a DME is showing that you're half a mile off, you can probably notice that, but a couple hundred feet off might not be noticeable (but is still plenty to be a big problem.)

so far, this is the only answer I've got with a practical explanation of the possible issue, with very well explained technical details (there were some a bit obscure points in the wikipedia explanation that were perfectly clarified here). At the moment this is the candidate to be accepted.

+1 for the amazing explanation. One question: Is there any checks done for ensuring the localizer is proper? It sounds like checking altitude is good enough for detecting issues with the GS only.

Instrument Landing System (ILS) frequences
Localizer: 108.1-111.95 MHz
Glide Slope: 329.15-335.0 MHz
http://en.wikipedia.org/wiki/Instrument_landing_system#Carrier_frequency_pairings_for_localiser_and_glide_slope

@Thebluefish To some extent, VOR and DME fixes can be used, but, as I mentioned in the answer, the precision there is not as much as you'd typically want, particularly on short final. Nowadays, GPS can be used. Someone more familiar with airliners than myself could probably provide better input, but I'd assume that the inertial navigation systems could also help between GPS/radionav fixes.

@rbp Yes, those are the carrier frequencies. You tune the carrier frequency for the localizer on your NAV radio and, on aircraft equipped for ILS, it automatically tunes the corresponding glideslope carrier frequency. As mentioned in the answer, in the case of both the localizer and the glideslope, those carriers are amplitude modulated at 90 Hz and 150 Hz.

I have pruned several comments. Please take any further discussion of citations/credentials/etc. to Meta - comments are really not the place for that sort of thing.

"[Source: I design RF receivers for a living.]"  Awesome .. I love the rare occasions you see sound info on the internet!  :)

Having worked as a software engineer on the lateral guidance subsystem of the FMCS (Flight Management and Control System) for the Airbus A310 about 30 years ago I found @reirab's answer fascinating.
I can fill in some gaps as to how the information from the different systems is used and why the ILS information is particularly critical.
On the A310 3 sets of input are used in determining the aircraft's position.
1) Every 200ms the inertial data from the set of 3 laser gyroscopes are used to basically say - "I was there. For 200ms I have been travelling at x knots therefore now I am here".
2) Every 1.5 seconds (in normal flight) data from up to 4 DMEs and VORs are used to accurately calculate the position of the aircraft.
3) During landing when ILS or MLS is available the ILS/MLS data is used in place of DME/VOR data.
There are two significant differences between 2) and 3). Because you are closer to the ground and precise vertical as well as lateral position is critical calculations are performed more frequently than once every 1.5 seconds. Because ILS/MLS data is much more accurate filtering of position adjustment is reduced.
What does that mean?
Well, when I do the radio position fix using DME and VOR I will also have an inertially calculated position. For various reasons beyond just not spilling the passengers' coffee I'm not allowed to correct this position by more than something like 5m (this is different if there has been no radio fix for a prolonged period of time). With ILS/MLS this filter factor is much larger, like 50m or even 100m, although the expectation is that such a large correction will only happen on initially acquiring the ILS/MLS signal.
Because such large corrections of the aircraft calculated position are allowed in Flight Management software when using ILS/MLS data it is critically important that there are no errors due to interference.

Hello Brian, welcome to Aviation.SE! That's a very insightful answer, I hope to see more of those!

it's "actual worthwhile information day" on the internet .. my God!  :)

You can read this related question if you want to learn more about the interference between electronic devices and airplanes. An answer there links to a very good document  written by NASA on the topic.
Long story short: Electronic devices are complicated. Airliners are complicated. Therefore, we can not predict exactly what the interference between them will be in all situations.
What your captain did happens frequently and is also covered in the relevant FAA FAQ.

Once your airline has shown the FAA its airplanes can safely handle
  radio interference from portable electronics, they can let you use
  your devices in airplane mode only most of the time. At certain times
  — for example, a landing in reduced visibility — the Captain may tell
  passengers to turn off their devices to make absolutely sure they
  don't interfere with onboard communications and navigation equipment.

So, the reason is that the captain wants to be absolutely 100% sure that there will not be any interference during such a critical point.

I'm kind of glad of reirab's answer, as this reads too much like (wavy wavy) it's magic and it might be (wavy) magic magic magic.

Adding to Reirab and Brian Towers' superb answers on this topic:
There are several ways an electronic device can interfere with aircraft operations, but the likelihood depends on age, aircraft design factors, and the nature of the EMI aggression (frequency, modulation type, and power -- there are also broadband EMI sources on aircraft, but we can safely disregard these for the purposes of this discussion, as PEDs generally are narrowband emitters unlike, say, an electric hydraulic pump which lost its interference suppression capacitors, spewing broadband hash as a result).
The typical concern, though, is of interference with an aircraft's sensitive navigation (ILS/LOC, G/S, NDB, GPS/WAAS, radio altimeter) and communication (HF, VHF, SATCOM) receivers (it's possible to interfere with the transponder/TCAS and weather radar signals as well, but those aren't quite the same level of concern as say the glideslope is), from intentionally emitting PEDs -- cell phones (especially GSM phones, as the GSM air interface is more EMI-prone than that a CDMA phone is) and 2.4GHz or 5GHz devices (WLAN, BT) are the main concerns on this front, although unintentional emitters (anything in airplane mode) are also a source of potential trouble.
Furthermore, comms interference takes an immediately noticeable form -- "what's that funny noise in my headset?" (Pilots of small General Aviation aircraft have  reported this on a regular basis.)  This is in contrast with interference to navigational receivers, which may be subtle but not immediately troublesome (losing a bird in a large visible GNSS constellation), obvious (a fail flag on a VOR or LOC), or insidious (a false glideslope indication).
However, the consequences are high here, especially during coupled or autoland approaches -- a human pilot can see that a full fly-down indication appearing out of the blue at 300' RA during an otherwise stable ILS approach is bogus and whack the TOGA button to get the crew some time to tell the noisemaker to zip their electronic lips, but an autoland computer won't know the difference and will follow it blindly.
A good piece of news is that in the somewhat-more-common case of an unintentional emitter (a device in airplane mode, for instance), the emissions tend to be simple, unmodulated carriers -- I see this as more likely to trigger a FAIL flag than a flight path deviation.  (I will put another question up on this topic, even -- it's worth asking about, and perhaps even an experiment or three.)
However, due to the large number of airline flights yearly (66 million according to ACI as per reirab's comment below), something that happens once in every 10^7 landings (Extremely Remote according to AC 25.1309) will happen every other month on average, and even something that happens once in 10^9 landings (Extremely Improbable according to AC 25.1309) is bound to happen every 15 years or so!  The reported PED interference instances are not nearly as severe in consequences as the 'worst case' of a full 'fly down' during the late phase of a coupled or autoland approach (or false radio altimeter readings causing a grossly premature mode shift during autoland), though, but the probability of PED EMI with aircraft systems on any given flight is enough to make it a concern -- 75 events were reported worldwide to the IATA from 2003 to 2010, and given this number, we get a probability of roughly 1 in 10^7 for PED interference on a flight.
From this, we can conclude that it would be highly unwise to take the chance of having a device talk all over the ILS signals during a critical phase of flight -- you would have to find a way to reduce the probability of PED interference with aircraft systems to 1 in 10^11 or better (this is 100 times less likely than what is currently allowed for a catastrophic event) in order to have a chance of challenging this wisdom.

+1 FYI, ACI reports that there were about 66 million airliner flights worldwide (at airports that report to ACI, at least) for the year ended Sept 2014. Since this issue is mainly a concern on landing, that's probably the most relevant figure to use for risk management statistics.

@reirab -- thanks, I'll rework that section!

Because they don't want to take any chances
Although most electronics have been shown to be unlikely to interfere with most avionics/communications equipment in most common circumstances the simple fact remains that we aren't totally sure yet. As RRR said, airliners and electronics are both very complex, and we can't predict exactly what will happen in every situation.
In a visual landing with instrument assistance, a small error is unlikely to matter too much - if you're a little fast you may get a bumpy landing, or an incorrect descent rate may cause a bounce or a go-around etc, but the landing is unlikely to be off by a significant enough margin to cause a crash. If there's a small deviation in course/speed/altitude it won't matter, the pilot will simply correct it - likely without even noticing he's doing it - or nobody will notice/care. If there's a larger error the pilot will almost certainly notice ("Wait, why is the runway over there?") and fix the problem
In an instrument landing, however, a very small margin of error in course/airspeed etc, present for 20 miles while the pilot has no frame of reference to know that it's wrong, can cause a major issue, because the cumulative error can add up over time and there's no way to know it's wrong.
Chances are, your phone will make absolutely no difference to the plane. But on that one occasion it does cause a problem, 300 people may be killed in a fireball. That makes "asking you to stop playing Angry Birds for 30 minutes" a very small price to pay for a a potential life-saving precaution.

I don't know that I would say that most electronics have been proven not to interfere. Shown unlikely to interfere would probably be a more accurate statement. Any device can cause interference, especially if it has some slight manufacturing defect (they only do strenuous emissions testing on a sample in the factory, not every device) or something has happened to it since manufacture (which could be as simple as a board or chip getting too hot or cold.) Otherwise, though, good answer. +1

Thanks, I've stolen your correction as it's a very valid point

@reirab You are 100% correct. In the early 2000s (well before "tablet computers" were everywhere), I worked for a company specializing in Electronic Flight Bags. Spinning hard drives were the norm since SSDs were new and expensive. A computer can put out an impressive amount of electromagnetic leakage. Most of the systems in the market at the time were not "proven" not to interfere. Gaining that "proof" was incredibly expensive, time intensive and mostly impractical. However, it was up to the individual pilot, as it still is, to demonstrate to himself that the EFB was "unlikely" to interfere.

My guess it that the watch on the pilot's hand is more likely to emit EMI than most modern gear is. But since it's still the pilot's responsibility, he's not taking any chances, whether they seem rational to everyone else or not. If he believes that people in blue shirts are a flight safety issue, it's his obligation to refuse to start the flight if anyone is wearing a blue shirt.

That said, I've also heard that asking people to put away electronics is a way for the cabin crew to secure loose items during the most critical phases of flight. If bad things happen, getting hit in the head by the iPhone from 27C would really suck. :-/

In fairness, if there's a big enough hole in the plane for an iPhone to fall out, the people below probably have more dangerous wreckage to look out for. I'd strongly dispute a watch giving out more EM then a phone or laptop, however, and remember there's a difference between one or two devices and 50-800 passengers

@JonStory I believe he was talking about someone in the cabin getting hit in the head by a flying iPhone (i.e. in the event of rapid braking or a crash,) not someone on the ground getting hit by it. Also, I agree with you regarding the watch. Phones and laptops do generally emit more EMI than a watch.

Fair point, but I maintain that if the plane has been thrown hard enough for the phone to be moving with any real force, you still probably have more important things to worry about.

@JonStory Not necessarily. Rapid braking in and of itself is not necessarily a problem. Also, turbulence happens and regularly projects all kinds of things through the cabin (people included.)

authors: Bob Jarvis - Слава Україні


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Do pilots have procedures and safeguards against unintended altitude captures?
Most advanced autopilots with a guidance panel allow you to select an altitude and fly to it.  It will often switch over to an altitude hold mode if the airplane is close enough to the selected altitude and the flight conditions like current vertical speed are right.  My question is about how vigilant the pilot has to be to prevent this altitude capture from happening at the wrong moment.
The Garmin G1000 manual describes the behavior like this:

As the aircraft nears the Selected Altitude, the flight
  director automatically transitions to Selected Altitude
  Capture Mode with Altitude Hold Mode armed (Figure
  6-7). This automatic transition is indicated by the green
  ‘ALTS’ annunciation flashing...
At 50 ft from the Selected Altitude, the flight director
  automatically transitions from Selected Altitude Capture
  to Altitude Hold Mode and holds the Selected Altitude
  (shown as the Altitude Reference).

However, this preselected altitude stays on as the flight proceeds, including during most mode changes.  There are some mode changes where this old altitude could cause problems if it initiates an altitude capture, like a transition to a go-around mode or an approach mode.  (For an example, see "A Sea Level State Of Mind" in this newsletter). The autopilot would try to reach the old altitude once conditions are right (which may require turbulence or other conditions to make the AP think it's trying to fly to the preslected altitude).  The plane would be doing what the pilot previously had told it to do, not what he was directing the plane to do at the moment.
From a human factors perspective, having the autopilot follow the old altitude target in a problematic scenario would make the system easier to understand, but would require more mental work from the pilot to maintain mode awareness.  Under certain conditions this altitude capture could potentially create a safety issue if the autopilot doesn't otherwise mitigate the sudden motion in an unintended direction.
Having seen a case where the pilot asked "why is that happening"  then realized he had mis-managed his altitude selector, I'm wondering about how smart the autopilot is and what the procedures are for managing the altitude selector.  Does the pilot usually expect the autopilot or flight director to ignore or clear the preselected altitude when necessary, or does he have to carefully manage the altitude selected to avoid an unintended maneuver?
EDIT: I'm asking if there are any safety features for altitude capture (like not allowing altitude capture while in glideslope mode) or checklist/procedures related to altitude capture.  If you think there aren't any of those and that the pilot takes full responsibility for any unintended altitude captures, please answer why you think so.

I'm not sure you are aware, but there is surprisingly little training required to fly advanced cockpits like the G1000, or for auto-pilot systems in general. So what the pilot "expects" to happen is typically based on experience. How "smart" the autopilot is depends on the autopilot, the G1000 being much "smarter" than something like a Gensys System 50.

Could you not just ignore the FD bars? From what you quoted it doesn't sound like the plane is going to do anything unless you tell it to

No 'vigilance' required; on a GFC700 AFCS, dial in your desired altitude using the ALT small or large knobs, then select your climb type, be either the vertical speed mode (VS) or a constant airspeed climb (IAS/FLC) mode.  Press the nose up/nose down keys to select the desired climb rate or climb airspeed and engage the autopilot.  It's that simple.  To change to a different altitude, simply repeat the process.

IANAP but from what I understand the flight director is just a visual guide for the pilot and does not control the aircraft. The settings are fed from the AP but unless the AP itself is engaged the pilot can simply ignore anything the FD is showing on screen.

@TomMcW On some autopilots the flight director and the AP are linked so if the autopilot is engaged it tries to follow the flight director.  On such airplanes any mitigation for these unintended captures would technically be in the flight director, not the autopilot itself.

@CarloFelicione Let me give some examples because in practice it may not be "simple".  Let's say we're flying at an altitude and make a maneuver by hand to avoid traffic or follow ATC.  When autopilot is reengaged it may try to capture the old altitude.  A more dangerous example would be if you tried to go-around and the autopilot tried to capture an altitude target (possibly below you) in the middle of the go-around (assuming altitude hold can be armed in a go-around, which isn't possible on all auto-pilots).

@CodyP, it will capture whatever altitude the altitude bug has been set to.  It doesn't care or select new altitudes based upon an arbitrary disengagement of the autopilot.  Finally, keep in mind you are flying the airplane; an autopilot simply does what you tell it to do, nothing more or less.

@CarloFelicione If you want, that comment could make a decent answer.  Just keep in mind that some altitude capture behaviors (like altitude capture after the glideslope has been captured) are not recommended by the FAA and incidents like India Airlines Flight 605 and a Brittania Airlines incident in Manchester on 6/21/1994 had pilots make mistakes with altitude selection.  Because of that kind of scenario, some flight guidance systems have limits on altitude capture like a minimum altitude limit and checklists may include a check that altitude capture is not enabled during approach.

@CodyP I guess what I was saying is that if you do a go-around or something the FD might not know what you're doing and show inappropriate directions, but unless you engage the AP it won't actually do it. If you engaged the AP without first selecting what you want it to do you could have a problem, but that would be pretty negligent on the pilot's part

@CodyP, under IFR when avoiding traffic you do want to return to previous altitude unless ATC gives you amended clearance. So the selected altitude remaining active is the right thing there and you treat any amended clearance as usual, dialing the new altitude in immediately (you should only engage autopilot once clear of conflict).

On the Boeing 737 NG:
ALT ACQ is always armed when flight directors and/or autopilot is engaged in a climb/descent mode (V/S, VNAV, LVL CHG, TO/GA, CWS P), it is however, as you suggest, inhibited in G/S and G/P (glide slope, glide path) modes.
On a typical ILS, I would typically set the missed approach altitude (which would be captured during missed approach in TO/GA mode) on glide slope capture. You could also set the missed approach altitude when ALT HLD is engaged, and you know that you will not need to change altitude again until when intercepting the glide slope. (When changing the selected altitude with ALT HLD engaged, the "held" altitude is maintained, until a new vertical mode is engaged, regardless of selected altitude).
HOWEVER, performing a Non-ILS approach in V/S or VNAV, the set altitude WOULD be captured on descent. To avoid this, the approach MDA is usually set on the altitude selector, and kept there until sufficiently (typically 300 ft) below the missed approach altitude. When passing through this altitude, the missed approach altitude would be set (logic is implemented to make sure the altitude is not captured during the "winding up" of the selected altitude).
Lastly, performing a Non-ILS using IAN (Basically Boeing's solution for performing Non-ILS with ILS display and control laws, using FMC data), the final descent would be performed in G/P mode, which behaves roughly like G/S mode, eliminating the use for initially selecting the MDA.

Out of the practical perspective: It is only the Non-ILS, Non-IAN case where this will be an issue due to "forgetting" setting MDA. Typical SOP: PM makes a callout n miles before the start of the final descent. In response, PF sets the MDA, possibly with verbal confirmation.

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text:
If I'm on a IFR flight plan with appropriate IFR reserves, and I'm given a lower altitude which will consume more fuel, what should I do?
Let's say I'm on a long-range flight at my requested altitude of FL250, and I am handed off into a very busy sector where the controller tells me "Descend and maintain FL210, for traffic."
At FL250, my fuel burn and capacity is sufficient to make the flight with better than IFR reserves.
But if I have to descend to a lower altitude, my fuel burn will increase and my true airspeed decrease to the point where I would have to stop en route for fuel.
How can I describe my situation to ATC in such as way as they will allow me to stay at 250?  Controllers do have some obligation to get you to your intended point of landing, and if giving you an altitude, airspeed, or vector that will adversely affect your flight, shouldn't there be some way to describe that?
What about saying, "if you give us 210, we're going to have to declare minimum fuel"

Descend as directed and start to figure out where to have your refueling stop?

@rbp Are we talking that with 4.000ft lower, you are having such a significantly lower groundspeed that you will spend your holding fuel, alternate fuel and contingency fuel and risk cutting into your reserve?

What are you really asking @rbp? You can request a different action but at the end of the day if the controller tells you to descend you do it. If that means you need to stop and refuel then the answer is obvious.

I'm not sure that this question is actually answerable in any definite way: there are plenty of things you could do, but it's hard to say what you should do unless you're actually in that situation. Or are you just asking for a list of all possible actions that a pilot could consider?

I guess I'm asking how negotiate for what the altitude I want. This isn't any different than, say, getting a 90 degree off-course vector, except in a different dimension. Does ATC have any requirement to accommodate me for a non-safety related request that will adversely impact the flight?

@rbp In my experience, the willingness of ATC to accommodate pilots varies widely from country to country. If I had a good reason for not wanting to descend, the first thing I might do is ask if they can give me a higher altitude (assuming I'm able to climb). I'd also want to know the nature of the traffic. If there's someone at the same flight level that I'm overtaking, can I just slow down. If there's someone overtaking me, can I speed up (if that's possible, but that would also burn more fuel).

@rbp - You may want to edit the question to add the clarification you have in the comment there.

Proceed as requested. You can glide to your destination (-: https://www.youtube.com/watch?v=sINyqmfbc9E

As I said in my comment, I don't think there's any single answer to this question because there are just too many variables and the scenario itself seems a little artificial.
But, the first thing you can do is negotiate with ATC. Accept the descent instruction but request a return to FL250 as soon as possible. Offer to fly at another altitude (including above FL250 if possible), or on an alternative route, or with a 10-degree course deviation or anything else that gives ATC options for keeping you at the altitude you want. Of course, there's still no guarantee that the controller will be able to allow it.
The next thing would be fuel management: depending on the aircraft and your current power settings, you may be able to sacrifice speed in order to gain range. But that might not help either if the end result is still that you won't have the reserves you need.
If none of those those things are possible then there's no good option except to plan a fuel stop. Declaring an emergency would be an abuse of the system and could lead to an investigation: you have plenty of fuel, just not enough to get where you want to go, and there's no immediate danger to the aircraft. (There's an NTSB ruling where a pilot declared minimum fuel, apparently in order to keep on a direct route. The FAA grounded him, he appealed to the NTSB and lost.)
In fact, you hopefully already planned for a possible fuel stop because if the winds are so different at FL210 and FL250 that it makes the difference between making it to your destination or not, that's something you should have found out during flight planning. But even if you didn't, or if the wind just changed suddenly, a diversion for fuel shouldn't be a big deal.

And don't forget that pilot planning to fly at FL250 should have some plan for what to do if forced down to FL100 by pressurization failure.

yes, that's an emergency, however. the scenario described is not.

If you encounter adverse conditions and you have to stop for fuel in a turboprop, its really no problem.

Ask if the traffic is ahead or crossing. If it's ahead, you can negotiate to remain at the current level by slowing down to 'his' speed/Mach no. If this is not an option, then I'd ask if I'd ask for a radar vector to avoid the traffic (possibly even a direct to YXZ) for separation - the controller will be able to see on her/his radar screen.
If still no joy, then you have no choice but to descend. Once down at the new level, ask when can you expect further climb - that way you can plan on whether you need to recompute your fuel or merely cruise for the next 20 mins (or whatever the case may be).
Finally, worst case scenario - lets say that you are stuck at your lower FL or altitude for the remainder of your trip knowing that you will not be allowed to climb, then your options are to request direct to ABC (short cut) and possibly make up some fuel at expense of arriving later due to your lower TAS/Mach no. or, get out the performance manual and calculate at that particular level/FL, what is your best endurance fuel flow.
Taking it further....now lets say you have exhausted all possibilities and ATC is still not accommodating you - now you can get an update of your destination's weather and assess whether at your revised ETA if the weather is still AT OR ABOVE landing minimums FOR THE APPROACH IN USE AT THE TIME OF ARRIVAL. If the weather is landing minimum or better, then you can legally continue to your destination using your alternate fuel as extra fuel.
The legality is to land with reserves, not alternate plus reserves.
You declare minimum fuel to ATC if at any stage of the flight, you anticipate that any other changes to the present flight route/conditions will cause you to land with less than reserve fuel. (note: this implies you now still have enough fuel to land with reserves intact. If you don't, then it is not minimum fuel you declare, but a fuel emergency)
So, if the weather at your destination is CAVOK and you filed IFR, since you are carrying an alternate, you are legal to use your alternate fuel after checking the weather (as previously mentioned).
If your destination weather is marginal....lets say it's RIGHT AT MINIMUMS you might find the weather whilst you are on the instrument approach reduces to below landing minimum as you pass the final approach fix...lets say you descend to 100 feet above the runway and cannot identify threshold markings, runway lights, edge lights - the landing environment [first 3000 feet (914 metres) of the runway], then you must perform the missed approach and assess whether you have enough fuel for another approach or merely fly to your alternate. This is an example whereby you would do what you can to preserve your alternate fuel as you may actually use it in the latter part of your flight.

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text:
What's the busiest airport that's not WGS-84 compliant?
Most latitude and longitude coordinates are defined relative to the WGS-84 standard, including GPS coordinates from your receiver.  However, some coordinates are specified using local datums, and these datums are often imprecise enough that even if you did convert them to the same datum as your GPS receiver, your receiver would be over a hundred feet off. (Code 7700 has more information on the issue).
Some examples can be found through Jeppesen and Garmin, like some of the SIDs around WIII and approaches at FLKK.  I don't have firsthand experience with any non-compliant navaids personally though so it's hard to tell how big a problem this really is.
Nowadays most (95%+) of navaids and airports are compliant.  I've heard even all international Russian airports are now compliant.  So what's the busiest airport with non-standard coordinates?

Can you give us an example of at least one airport - presumably not very busy - that isn't WGS-84 compliant? That might help to clarify what you're asking.

@Pondlife, just follow the link. Apparently such problems are real, though really strange to me, since they say “Although the fidelity of their methods are not in question, the simple fact that using different methods exposes the possibility that position information may not match when compared to WGS-84 methods.”, but if the fidelity was there, matching the coordinate systems and converting between them might be tedious, but still possible and reliable.

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text:
Are there two navaids on the same frequency close enough to be confused?
Several RTCA DO standards require checks for bad navaid tuning including, "In lieu of using ground-based navigation aid designated operational coverage (DOC), the system should provide, checks which preclude use of duplicate frequency navaids within range" (DO-236B for RNP). Are there example locations where you can be just outside the range of one navaid and pick up another on the same frequency? Has anyone seen potential for confusion like this in real-life, possibly with complicating circumstances like IRS drift?
I thought most navaid frequencies were chosen to avoid potential confusion, but an article in Skybrary seems to suggest this is possible when operating on the boundary between two designated operational coverages, although it may only be talking about voice frequencies.  There's also an anecdotal account of VOR and DME receiving from different stations that share a frequency by Tom Rogers at AVweb.  There's a similar question on here explaining navaid spacing, but it doesn't answer whether and under what cirucmstances two navaids could be confused for each other.
I even spent a few minutes looking informally through a navigation database couldn't find any examples of close navaids using the same frequency (not including non-colocated systems where each part operates on the same frequency). The closest I got was in Michigan where the Selfridge Air National Guard Base localizer and Capital Region International Airport 28L localizer both transmit on 110.10 MHz. They're less than 100 miles apart but both are ILS localizers, so you'd never confuse one for the other without already being seriously lost.  Additionally, Mendoza and Vina del Mar's VDM share 114.9 MHz, but there's the Andes Mountains in between so again you'd extremely unlikely to confuse the two.

Short answer
ICAO recommendations include a protection for same-frequency navaids, restricting the received power of undesired stations to a level preventing interferences. To meet this restriction same-frequency stations must be considerably distant.
The result, in normal conditions, is the receiver cannot switch uninterrupted from the current station to a same-frequency station, there will be a significant period where none of the stations can be received.
However distant stations signals can be significantly increased by propagation perturbations like sporadic E propagation or wave ducting. These instable conditions can break the co-channel protection plan and increase distant stations levels above the reception threshold, possibly above the local station level. When this occurs the receiver can wander between stations unnoticed if the station ID is not checked.
I'll detail the answer for VHF navaids, but it can be extrapolated to other navaids.

ICAO co-channel protection

Are there two navaids on the same frequency close enough to be confused?

The normal range for VHF is the radio horizon, that is 15% further than the geometrical horizon (k-factor = 4/3).
Same-frequency navaids are spaced well further than this horizon and by measuring the signal level rather than the distance. Using the signal level allows to take into account directional transmitters, like ILS components, where energy is concentrated in some solid angular sector.
At any point within the service volume of the desired VOR, the ratio between the reception level of the VOR and the reception level of any same-frequency VOR must be greater than the co-channel protection factor:

Co-channel interference for a VOR, source
This ratio is  20dB (= 100 times) for ILS/VOR, and 15dB (32 times) for NDB. More information in ICAO Doc 9718, chapter 9. Note this ratio is the recommended minimum, but if for some reason this ratio is not effective, then a solution must be found, ICAO/ITU have procedures to work exceptions.
When the desired VOR transmits and is within range, the receiver reduces its amplification to the bare minimum and the undesired signal, given the protection factor, is undetectable. When the receiver is located at the limit of the service volume, the other signal must still be 20dB weaker. It means the two VOR cannot have their service volume next to each other, but must be significantly remote.
But there are circumstances where propagation can be boosted, and distant stations behave like if they were considerably closer they actually are.
Propagation phenomena

Are there example locations where you can be just outside the range of one navaid and pick up another on the same frequency?

Some conditions of propagation allow reception of signals well beyond their usual limits of reception:

Night effect, effective under 30 MHz, may affect NDB. This perturbation is common, but its exact result is unpredictable. See What is the night effect?

Tropospheric ducting, due to a temperature inversion in troposphere, mostly applies to the lower VHF band. As refraction index varies with temperature, a tunnel is created between two air layers and the signal is ducted like light in a graded-index optical fiber.

Sporadic E, which affects VHF, is a situation where a signal is reflected by ionosphere and can be received at distance larger than 2,500 km.

In polar regions auroras are ionized curtains created by solar winds which alter propagation in unpredictable ways.

The actual use of the frequency, navigation, data, voice, has no significant impact.
Consequences regarding co-channel protection
With this in mind, the coverage of a navaid is not limited by a circle of some radius, you can be at a location where two navaids at very different distances are received with an equal power, or where the remote navaid is stronger than the local one.
Such super-propagation events are more frequent in HF band, and the night effect is present each night. VHF/UHF events are seen during short periods of time, between 10 and 30 minutes a few times per month, more often around summer solstice.
ID check is required before signal use
Communication cannot be protected against these effects. The possibility to receive a remote navaid stronger than a local one can never been eliminated, even with a careful planning of the radio services and ground stations.
That's why when lives are at stake, identifying the actual navaid received is mandatory. Note the protection factors listed at the beginning already include a 6dB (4 times) margin specific to aeronautical communications, not used for common spectrum planning.

Excellent answer! Do you have source for "Sporadic E propagation [...] is tightly tied to solar activity"? I tried to find some more info on sporadic E and most sources say it is related to season ("Sporadic E activity peaks predictably in the summertime in both hemispheres.", Wikpedia). I found one paper showing an 11-year periodicity though.

@Bianfable: Nothing escapes your vigilance! I'll update the answer as it seems incorrect. If you want more details of the overall mechanism, this radioamateur paper looks a very good summary. Layer E propagation is linked to seasons because there are more Sun light, and longer during Summer.

#1 I don't know.
#2 On December 20, 1995 American Airlines 965 crashed into a mountain top near Cali Columbia when it was flying down the the valley between mountains and the 757 autopilot turned the aircraft left to follow the loaded flightplan...  the flightplan was loaded with an NDB called "R".  The problem was that the "R" they loaded into the flight plan was the "R" in Bogota, not the "R" NDB on their approach in Cali.
When the aircraft turned they were talking about rest rules and duty times and flight attendants and had one of those "What's it doing now moments".
The GPWS activated to warn of the mountain they had just turned towards on their approach. In the confusion while executing their evasive climb maneuver they forgot to retract the extended spoilers, which sealed their fate.
After this accident Boeing made it so that spoilers automatically retract with climb power added and Jepessen made sure that there were no longer any single character navaids.
It seems relevant to your question.
Maybe?
Relevant to being a pilot:  Monitor the autopilot and your progression on the chart/map so you always know where you are and are ready to disc A/P and fly the airplane without having to try to figure out where you are first because you aren't paying attention.
Never have a "what's it doing now" moment.

Yes, that does provide an answer in that people have died due to trying to navigate to the wrong navaid. A range check could prevent such an issue, and interestingly it's not even that a valid signal was being received from the wrong navaid.

Your example is a confusion of navaids because of the same name, not the same frequency. Not sure how that answers the question...

"#1 I don't know." was my indication to the reader that I was not answering the question.  I wasn't sure that my comment was completely relevant either (which is why I said "Maybe"). ;)   Consider my comment to be free flight instruction.  I usually bill at $75/hour.  :)

After reading an analysis of the accident it look like while the poor pilots bear the final responsibility of using the wrong NDB, the road was well prepared by the way the NDB were entered in the FMS database. The indexation principle provided by ARINC-424 is itself a dangerous design: two NDB cannot have the same name "R" in the database, one must be given a fictitious name, here "ROZO", though if the pilots don't know how engineers renamed it, it won't be displayed when keying "R", but only when keying "ROZO". Purely stupid.

@ymb1: I didn't see this was the same frequency in the report, I think it's an error, else two NDB with the same frequency and the same ID 'R', this is just impossible to work with. My understanding is they had a different frequency, and while they had the same ID (so checking was useless) they were not coded in the FMS under the same name R (ARINC/FMS limitation), Romeo (Bogota) was coded R, Rozo (Cali) was coded ROZO. The crew selected R because this was the ID on the approach plate. They didn't remember Rozo was coded ROZO and ROZO didn't appear in the FMS navaids list when keying only 'R'.

@ymb1: Ah so the Romeo NDB signal was not received, but the FMS used the known coordinates as a waypoint and changed the route?? That's even more crazy and actually not related to the question, the frequency didn't play any role.


text:
What's usually going on in a "hot" military operations area (in the US)?
Like many people in aviation, I've been around pilots who've flown in or through the edge of a Military Operations Area (MOA) while operating under visual flight rules. Usually they ask ATC if the MOA is hot, and sometimes the answer actually is "yes".  In that case, the pilots stay clear for safety.  My question is, what's usually going on in the MOA at the time and how bad would it be to enter it in VFR?
There are few reputable sources on what typical operations are in military operating areas.  This Global Air blog is the best explanation I could find, although this post isn't a bad source.  Those sources seem to err on the side of exaggerated caution though, describing "lights off" operations, practice dogfights, and 500 KIAS speeds.  Are those kinds of operations daily, or evenly weekly, occurences in military operating areas?  Where on the spectrum from the mundane "practicing approaches" to dangerous "simulated high speed dogfight operating heads-down" do most military flights lie? Are operations consistent or do they vary from place to place and month to month?
Background information for those outside the US:  MOA's are defined in the Aeronautical Information Manual (AIM), paragraph 3-4-5, as well defined airspaces near military bases "for the purpose of separating certain military training activities from IFR traffic."  They are different from restricted zones with particular defensive significance or bomb and missile activity.

Just as an FYI, many restricted areas have times when they are hot and you can’t fly through them and times when they are cold and VFR and IFR traffic can fly through them. Vandenberg FB, where they just launched the SpaceX rocket, has four restricted areas. The area around the launch site is always hot, but the rest  are usually cold. They turn hot when launch preparations are underway or other activities are ongoing.

Military pilot here.
I can certainly understand why there's confusion surrounding MOA's, having been a Private Pilot long before I flew for the military, and I'm glad you asked the question.
The second post you referenced is exactly right. It is extremely common (at least in our training areas) to have  dogfighting and other tactical training with airspeeds above 500 KCAS, and altitude changes of thousands of feet in seconds. No one uses a MOA for instrument approaches. This type of flying occurs multiple times per day, and it's extremely frustrating when civilian aircraft fly through the airspace without talking to ATC. We generally don't have a lot of time to work in the area, so we have to maximize the training we get. Stopping a fight/intercept/practice bomb run because a Cessna wanted to transit the airspace generally results in a reset of the entire scenario and wastes countless dollars and training opportunities.
If there's an emergency we will, of course, understand and please do whatever keeps you safe. Short of that, please, please get flight following, and do not fly through a MOA.

Lemoore AFB is in the California Central Valley and the MOA is large. I have flown through the Lemoore MOAs many times and watched fighters in a dogfight appear to literally fall out of the sky—descending 10,000′ in no time at all. Usually when ATC calls out traffic for you it is something like, “Traffic at 2 o'clock 5,500', 5 miles.” The call for these guys is more like, ”Traffic at 12 to 3 o'clock, 12,000' descending to 3,000', 5 miles.”. When it is hot, Lemoore Approach hands you off to the controller directing the activities, but I forget what they are called.

Military Training Route
I was on a low level going south down the Florida panhandle. At some point the flight would turn back north, and eventually fly over Lake Okeechobee. That was my favorite part of the flight, fast and low over the lake. Our low-levels were usually done at 500 feet and 360 knots. That speed did make navigation easy since it was at 6 miles per minute.
During the flight south there was some motion in my peripheral vision, and out of nowhere a light single engine passed by my nose at co-altitude. Wow! That was a close call.
 Miltitary Training Area near Midair
We were involved in a great exercise within the Tyndall MOA. The USS Nimitz was on the east coast of Florida and we were to practice inter-agency operations around Tyndall AFB. Our mission was to hold at a point near the coast and wait there for instructions from our Forward Air Controller (FAC).
The FAC, who was in a light aircraft in the target area, called us in to the target after a short wait. These were targets of opportunity. We were low at 500 feet and very fast. The target was in front of us when the FAC called, "Hit my smoke." He had launched rockets with Willie Pete (white phosphorous). We popped up in a loose combat formation, and I was on the wing. We delivered our weapons and were egressing to the coast.
I had never seen this before. As we pulled out I saw a long line of tanks along  a straight dirt road that seemed to go on forever. There were troops on the road. It was a massive movement of equipment and personnel along the side of the road. I couldn't resist the temptation to take a look.
I peeled off my leads wing keeping him in sight as I lost altitude. I was approaching a long line of tanks and the treeline approached the horizon. When the trees had been replaced by the sky, I leveled off. There was a minimum altitude for tactical jet aircraft of 500 feet, and instantly I saw the side door of a Huey. I didn't see the Huey, but only saw the side door. It came out of the trees. I instinctively pulled back hard on the stick and rolled back towards my lead who was departing the area.
To this day I still remember the crew member standing in the door, able to see the visor in his flight helmet. Not sure what was going through his head when he saw me coming at him. From that point on I was very careful operating within MOA, realizing that there were all sorts of people and equipment there, each with there own space for safety.

"Hot" simply means active (which ATC will know), you can find the start and end times here if you are curious. Its more than likely live firing, low altitude high speed operations, paratroop training, or anything else the military decides they need a block of airspace for.
According to this SB (p. 138)

Activities.
MOA's are established to contain certain military
  activities such as air combat maneuvers, air intercepts, acrobatics,
  etc.

you can also find a list of all MOA's with time frames in the above linked MOA's. For various reasons the military is unlikely to disclose exactly whats happening. Its generally wise to stay away from these areas.

Concerning your example from SC:  in that example the pilot was only practicing approaches, so I wonder what the implications are for the "are typical operations in a MOA dangerous maneuvers?" part of my question. I'm also not sure it was in a MOA since it happened NE of Charleston and Berkley county airport "near the west branch of the Cooper River" at 1,400 to 1500 ft.  The NTSB initial report doesn't mention it being inside an active MOA.

It was not an MOA it was a military training route but the risks are similar in that both areas may include low altitude high speed maneuvers.

@Dave -- actually, re: Moncks Corner -- the F-16 was in the IFR system, on an instrument approach to KCHS.  (ATC attempted to give the F-16 vectors for the traffic, too!)

Live fire exercises will typically occur in a Restricted area vs a MOA.  At least all the targets I am familiar with.


text:
Can planned runway closures prevent a plane from leaving an airport?
I was on a flight recently where, due to the long runway at my airport being repaired, we had to fly to another nearby airport to get enough fuel for the rest of our trip.  We couldn't take off with enough fuel for even a two hour flight because of the runway closure, and this was at a major GA airport.
I'd imagine there are runway closure regulations to prevent this kind of thing from causing serious logistic issues, but do planned runway closures ever make a trip impossible even with refueling?  Considering factors like airspace restrictions, distance, heavy cargo, etc.  it's easy to imagine that a flight normally possible could become impossible due to a planned runway closure, even if you try to refuel at the nearest allowed airport.  What airport regulations are there to mitigate this and how big of an issue is this? For example, can an airplane ever be stranded while airport construction is occurring? How are airports required to notify pilots in advance of planned closures and are there regulations to prevent stranding airplanes or causing logistic issues by closing the only long runway at a critical airport?
Note: I'm not concerned with weather-related closures here, but rather planned runway closures.  My question is aimed at GA where things are more flexible, but applicable answers from commercial aviation can be accepted too.

Yes, of course. Aircraft have minimum runway lengths that they can use, if there are no runways available that are long enough, then you have to fly to an alternate or cancel the trip (mission). Some GA airports only have a single runway, and of course if you can't land on it, you can't go there.

@RonBeyer if you can provide relevant regulation, statistics, or even examples then that'd make a good answer. It's obvious this can occur in situations like really heavy cargo or short runways, but I'm  wondering what factors there are to mitigate this (including how often there's a reasonable alternate for landing or refueling) and how big of an issue it is.

What regulation are you looking for? That you can't land on a closed runway? Or that you have to land on a runway that meets the distance required by your aircraft? For example there is a small airport near me called Carter (92C) that has a single paved runway and a turf runway closed in the winter. If the paved runway is closed (for example, when it is being plowed), you can't fly there. Every pilot should always have a viable alternate before taking off though, regulations 91.13 and 91.103(b) apply.

@RonBeyer I'm looking more for airport regulations rather than pilot regulations because pilot regulations here seem straightforward.  I added to my second paragraph to address your question.

Pilots are notified of abnormal airport conditions—such as runway closures—via the Notices to Airmen (NOTAM) publications.

There is actually a really good example of this...
In 1994 it was decided that the infamous Meigs Field would be demolished and turned into a park. (I say infamous because for all of us young flight simulator aficionados this is the home airport of Microsoft Flight Simulator).
On a Sunday night, March 30, 2003, the Mayor Daley of Chicago ordered the runway bulldozed by gouging huge X's into the runway itself. This was done in the middle of the night with many aircraft still based on the field. No notice was given to the FAA or the owners of aircraft still on the field. In fact, an inbound flight had to be diverted by air traffic control because of equipment on the runway.
You can imagine what kind of excrement storm this stirred up. In the morning of March 31, 2003 at a press conference Daley defended his actions by saying that it would have saved the city of Chicago years of court battles over closing the airport. Luckily the aircraft at the field were allowed to depart from the 3000' taxiway that ran adjacent to the runway.
In this case, the worst the FAA could do was fine the city of Chicago \$33,000 (\$1,100 per day) for closing an ILS procedure without a 30 day notice. Later it was found that they misappropriated FAA funds to demolish the runway and build the park and that had to be paid back, but there were no legal implications of this. The airport was not under federal funding and the city was technically within its rights to close it.
You can read more about it on Wikipedia
The point is that airports are often owned by the city in which they reside, or sometimes the state, at least in the U.S. The FAA releases many guidelines for airports to make them safe, and even has an "airport certification program" for certifying part 139 operations, however these are done through "orders" and guidelines, not much in the way of FAR's.
ATC, if federally operated, must comply with the regulations that apply, including giving NOTAM information for planned closures, or even NOTAM's for closures that are not planned, such as runway debris or accidents closing runways.
So really when you get down to it, the city that owns the airport can close runways (or the entire airport) whenever they want, and yes this may strand aircraft. Usually if the airport is closed unexpectedly and aircraft are still there, they will be allowed to take off from taxiways or grass strips. Worst case scenario the wings will have to be removed and it will need to be trucked out, at least for GA operations.
Depending on the Ops Spec for the airline, it may also allow the aircraft to take off from a taxiway. Usually something like that is done without passengers on board though.
So to address your specific questions...
Is an airport required to give notice to pilots (or aircraft owners) about planned construction? No, there is no requirement for this. Usually what happens is airports don't give adequate notification and it agitates owners, but it all works out in the end.
For example, can an airplane ever be stranded while airport construction is occurring? Yes, at Meigs originally owners had to fight to be allowed to take-off. Many aircraft where going to be scrapped as they were too expensive to move by truck, and owners didn't want to pay. The city didn't want to pay, and the insurance company didn't want to pay, so that would have kicked off a big set of lawsuits for the city.
Closing runways can't always be avoided, for example when there is an accident, weather (flooding, snow, ice), earthquakes, etc. It happens and the system "works" around it when it does.

That is the best example ever of fait accompli.

One note is that the FAA often has contracts with cities requiring the airport to be operational for a period, which is one of the few ways the FAA has to prevent airports from closing all runways. This is one of the issues being brought up currently (in 2016) as the city of Santa Monica is trying to force closure of their airport.

Dick move, Daley.

It shouldn't happen if people check their NOTAMs.
The Australian rules are found in the Part 139 Manual of Standards. I don't know where to find the US equivalent but I'd be surprised it it is vastly different.
Firstly, if the aerodrome must be altogether closed for non-emergency work, a NOTAM must be published at least 14 days beforehand:

10.10.2.8    The operator must not close the aerodrome to aircraft operations due to aerodrome works, unless a NOTAM giving notice of the closure has been issued not less than 14 days before closure takes place

For other work of a significant nature that doesn't require the whole aerodrome to be closed, a Method of Working Plan (MOWP) is usually required. This is basically a comprehensive plan outlining what work will be done and the impact it will have. It is issued well in advance of the works being undertaken to local operators, ATC, airlines and other stakeholders.

10.11.3.1    The MOWP must:
  (a)      include an outline of the full scope of the works and state which aerodrome facilities are affected.
  (b)      provide the planned date and time of commencement, the duration of each stage and the time of completion.
  (c)       contain the following sentence:
  “The actual date and time of commencement will be advised by a NOTAM, to be issued not less than 48 hours before work commences”.

Some basic repair or maintenance activities are classed as time-limited works. These don't need to go through the MOWP process, however they need a NOTAM to be issued 24 hours in advance if they affect safety and cannot completely pack up with 10 minutes notice. This includes repairing lights, mowing the grass, even sweeping.

10.10.3.3    A person must not commence time-limited works that require more than 10 minutes to restore normal safety standards to the movement area and remove obstacles, unless a NOTAM has been issued not less than 24 hours before the commencement, giving the date and time of commencement and the time required to restore normal safety standards.

The key theme here is that unless it is an emergency, every repair work affecting aircraft operations is required to be communicated well in advance, via NOTAM and a direct approach to operators.
Also, as most airports are private enterprises they understand the value of not screwing over their customers, so for big-impact works they give a lot more notice. When Dubai resurfaced one of the runways a year or two ago, I recall reading about it over 6 months in advance, because it was would cause such disruption. This generous notice period allowed their customers to alter their schedules to minimise commercial impacts.


text:
Whats the derivation of a low speed ratio for stall warning?
I've seen an equation related to stall and angle of attack in a few places like this patent that goes more or less:
$$ \frac{CAS_{alert}}{CAS_{current}}  = \sqrt{\frac{C_{L,current}}{C_{L,alert}}} $$
Where CAS is your airspeed and C_l is the coefficient of lift at certain angle of attack.
It seems intuitive, for example if you are in unaccelerated flight and using 90% of your available lift then you are about 5% over your alert speed. However I'm confused about a few details like why there's a square root in the equation, or whether there are scenarios where this is just an approximation and it breaks down. Where does this speed ratio come from?

Not sure the complete context of your question but much information can be found at https://www.ecfr.gov/current/title-14/chapter-I/subchapter-C/part-25/subpart-B/subject-group-ECFR1cd58fc7d4bfee0/section-25.207 for part 25 certified aircraft.
With regards to your equation, it does correctly reflect the proportionality between CL and airspeed for 1-g level flight with the caveat that technically, it's for Vtrue, not Vcas.
{CL = W/(rhoVtVt*Sref/2)}

$\rho$ times $\text{TAS}^2$ is approximately $\text{CAS}^2$. (More accurately it's equivalent airspeed squared).

Ve = Vt * sqrt(sigma) = Vt * sqrt(rho/rhoSL)

It's a simple scaling law. The lift is proportional to $C_L$ times the square of the equivalent airspeed. In other words, for a given load factor, $C_L(\text{EAS})^2$ is a constant. So $\frac{\text{EAS}_1}{\text{EAS}_2}=\sqrt{\frac{C_{L,2}}{C_{L,1}}}$ is true for any two flight conditions at the same load factor.
At lower mach numbers equivalent and calibrated airspeeds are the same so they are often used interchangeably.


text:
Are the first solo flights by a student pilot more dangerous?
The first solo flight is a nail-biting moment, not only for the student pilot, but also for the flight instructor who's sending him off.  In theory not having a CFI reduces the margin of safety compared to flights with an instructor (at the same point in a student's training).  I've also heard stories about mistakes and even crashes on first solo flights, and according to "Crashes of instructional flights" by Baker, Lamb, Li, and Dodd, solo flights account for about half of instructional flight crashes.
Yet even though the biggest safety measure- a CFI- is gone, are these student solo flights really more dangerous?  Are there any studies on accident rate among low time GA pilots vs (solo) students?  Also, are incidents more common in the first solo flights than at other parts of a student's pilot training?

There are probably studies somewhere but anecdotally, I know that the highest accident rates occur somewhere between 75 and 200 hours when confidence and ego can outweigh wisdom and learning.  Your license is a license to learn.

A student on first solo is probably closer to sustained real training, more aware, head full of the instructors wise words than they will ever be again and the instructor is obviously convinced that the student is ready and capable.  My reasoned guess is that accident rates on first solos are therefore low.  I also guess that many make mistakes and quickly learn.  I did. The CoG change in an R22 on lift off with no talking ballast in the left seat is a real attention getter!

I would second @Simon 's statement. My first solo flight, I recalled what my CFI said to me in every phase of the flight, the only thing that was different was that I was suddenly climbing with a higher rate of climb due to less weight. I only allowed myself some distraction during cruise flight in the pattern ( I was alone), but take-off, approach, flare and roll-out, I was sharp as a knife...

A (sensible) instructor would have the student flying at least a few patterns before stepping out of the aircraft, therefore the student has already "warmed up". I'd argue that the first flight after 9 months of not flying, without an instructor, is more dangerous.

seconding (thirding?) Simon's statement. Unoficial stats and word arround the cooler is that 75-500 hours are the most dangerous. First solo is probably one of the safest flights you'll take

My experience is the same as above - while very nerve racking, your FIrst Solo is very much a "paint by numbers" affair. You're fresh off the press, you know your routines backwards and you can almost hear your instructor speaking to you. Additionally, it's a VERY short flight - a single circuit if all goes well. There's also very little time to think - by the time you're thinking "Holy crap, I'm by myself" you're coming upto to your first checks and then all of sudden you're back on the ground!

The relationship of experience to safety is complex and studies can be contradictory.  Most research indicates that inexperienced pilots and students are less likely to have fatal crashes (Bazargan & Guzhva, 2011; "Correlates...", Li & Baker).  A few tangential studies indicate pilots with less experience are the most dangerous ("Accident Rates for Novice Glider Pilots...", Jarvis & Harris). Some indicate a more complex relationship  ("Instructor Perceptions... Glider Pilots", Jarvis & Harris; "Factors Associated...", Li & Baker). See also "Crashes of Instructional Flights", Baker, Lamb & Dodd

I always wondered if CFI was a particularly dangerous job. The answers here indicate that it's actually not as dangerous as a regular pilot. Go figure

The thing that seems to cause most accidents is over-confidence, which leads people to carelessly cut corners, which leads to a general apathy to things they are doing that might be dangerous.  When you first fly, trust me, you are paying attention to everything.  It's not the most skilled you'll ever be, but it's probably the carefullest you'll ever be.  At that point you just need to wait until you actually think you know what you're doing before really screwing things up.

There is no evidence to back the claim that the first solo ride of the student is more dangerous compared to the later ones.
US NIH conducted a study on the accidents with solo pilots, which doesn't indicate that first-time solo fliers are any more prone to accident than others. Aircraft accidents with student pilots flying solo: analysis of 390 cases by  Sjir Uitdewilligen and Alexander Johan de Voogt analyzed NTSB probable cause reports of 390 crashes that occurred in the period 2001 – 2005, concluding that,

Student pilots flying solo show fewer injuries and fatalities compared to general instructional flights while in our sample first-time solo student pilots did not feature any fatalities.

Note that this gives only the injuries and not the times the students got into accidents per se. Out of a total of 3811 accidents involving student pilots, 390 occurred while they were flying solo and around 50 involved first-time student solo pilots. Actually, student pilots themselves are prone to less accidents compared to others. From Comparative Analysis of Accident and Non-Accident Pilots by David C. Ison:

Most accidents (49.1%) were conducted with individuals holding a private pilot certificate. Second in incidence were commercial pilots (28.2%), followed by Airline Transport Pilots (ATPs) (13.7%), and student pilots (5.7%)
Considering that 20% of pilots hold a student certificate, these individuals have a disproportionally low accident occurrence.

The report also gives some additional insight into the reason the first-time solo fliers have lesser injuries:

... first-time solo pilots are commonly confined to the airport and practice their takeoff and landings. Such operations may result in accidents, but they will occur near to the ground with a lower risk of a fatality

The report also gives some data about higher experienced student pilots sustaining more injuries, though nothing conclusive, noting that,

... in 25 cases, student pilots were reported to have more than 100 and up to 322 h of flight experience. In the dataset, these pilots were significantly more often injured than students with less hours of flight experience.

It has to be noted that as the hours logged gets more, the students get into more demanding flights which may be reason for this increase in injuries.

The statement you cited about the lower risk of fatality for typical student flights is supported by the statistical analysis in Li, Baker, and Grabowski's "Factors Associated with Pilot Error in Aviation Crashes," which states  "the lower case fatality rate in student pilots [even after adjusting for pilot hours] was due to the fact that their crashes occurred disproportionately near airports, during daytime (6 a.m. to 5 p.m.), under visual meteorologic conditions, and without aircraft fire."

"Considering that 20% of pilots hold a student certificate, these individuals have a disproportionally low accident occurrence." They also almost certainly have a disproportionately low number of flight hours in a given time period. The discrepancy between occurrences for pilots with PPLs and students is much more likely simply due to the former flying a lot more than the latter rather than being less safe. The students who do fly frequently usually don't remain students very long.

I couldn't find any analysis of first solo flights, but based on NTSB accident reports it looks like accidents are very rare. There are only 60 reports of fatal part 91 (GA) accidents with the words "first solo" in the report, and 492 non-fatal accidents. But many of those are false positives anyway, because they include reports with things like "with his first solo occurring about a month prior to the accident", so the real numbers are lower.
Training flights in general are very safe compared to other flights. In 2015, student pilots accounted for only 6.5% of non-commercial fixed-wing GA accidents. That's much lower than private (47.1%), commercial (26.6%) and even ATP (12.2%) certificate holders.
AOPA has a detailed report called Accidents During Flight Instruction that comments on student solo flight in general, but not on first solos specifically. It says:

Two-thirds of all fixed-wing training accidents come during primary
  instruction, and two-thirds of those are during the relatively few
  hours of solo flight by student pilots. However, fatalities on student
  solos are extremely rare.

And it does give some information about your question on first solos vs. other phases of instruction:

Two-thirds of all fatal fixed-wing accidents occurred during advanced
  instruction, less than half of them while pursuing a specific
  certificate, rating, or endorsement. Transition training, flight
  reviews, generic refresher training, and specialized instruction in
  areas such as mountain flying, aerobatics, and cropdusting
  collectively accounted for over 60 percent of all advanced dual
  accidents, including more than half the fatal accidents.

Other people have already commented on why student solo flights in general are relatively safe: the student is primed with plenty of recent training and instructor feedback, and the flight conditions are well controlled.

Some of the statistical logic in this answer is unsound. e.g. "In 2015, student pilots accounted for only 6.5% of non-commercial fixed-wing GA accidents. [thus showing that student flights are safer than nonstudent]" - but you didn't normalize to account for the fact that many more flight hours are flown by nonstudent than student pilots.

@pericynthion Great point, and I'm the first to admit that my statistical logic isn't always sound :-) Those figures are indeed the number of accidents, not the accident rate. But to take one comparison, the FAA stats say there were 20% more active student pilots in 2015 than commercial ones, and the numbers above say there was 1 student accident per 4 commercial pilot ones. So fewer commercial pilots had significantly more accidents, and it would take a huge difference in flight hours to even that out.I think the numbers are at least indicative of a lower student accident rate.

@Pondlife "and it would take a huge difference in flight hours to even that out" There almost certainly is a huge difference in flight hours between commercial pilots (who are frequently professional pilots) vs. people who hold a student pilot certificate. Student pilot certificates remain 'active' for several years even if the student stops flying, so many of those student licenses likely belong to people who haven't flown at all recently. Frankly, I'd be surprised if the number of hours/pilot/year for students is more than 10% of that for CPLs, likely it's much less.

No.
Anytime you get on a car or airplane or any mode of transportation, it is dangerous. Yet most of those journeys end without any incident.
First solo is a very short flight, mostly of a few circuits of the traffic pattern. The instructor is watching and talking to the student. The student is well trained at that point and literally know what they are doing.
I think Simon's comment is very important and needs to be remembered, most accidents can happen when confidence and ego can outweigh wisdom and learning.
You should also notice that most of student's flying occur in VMC or better conditions. This includes both first solos and checkrides. Being the pilot in command, you can cancel the flight if you think any condition is not suitable.

I agree, but this answer is just opinion and the OP is asking for evidence and statistics.

@pericynthion What statistical parameters can we use as a measures of confidence, ego, wisdom and learning?


text:
What does "synoptic" mean in avionics?
I came across a mention of a display of "graphical aircraft system synoptics" on this page.  I think in context that it means "status display" or "summary page"- the page that shows flap position, hydraulic pressure, etc.
However, synoptic isn't even a noun in some dictionaries. Google searches for the word turned up only passing references to the term, not a solid definition. The actual word usage seems very aviation-specific.  What does the term actually mean and how does it differ from "system status" or "summary page?"

In this context synoptic would not be a noun, as it describes what something is like, making it an adjective.

Synoptic is an adjective form of the noun synopsis

@Jpe61 In real world usage like "system snoptics", "Subsequent Boeing aircraft (B-747-400, B-777) do incorporate synoptics", or "aircraft synoptics and electronic flight charts", it is a noun, not just an adjective, as it comes after another noun.  (Although use as an adjective also appears sometimes).  Compare similar adjectives like short, concise, or abbreviated.  Could those words be substitued in, e.g. "system shorts", "incorporate concises", or "aircraft abbreviated and flight charts"

@crasic Sounds like you have a good start to an answer there.  If you could move that to the answer section and elaborate on what it means in sentences like The Boeing 757 cockpit does not provide subsystem synoptics, it would be helpful.

The only place I've ever encountered this word was in Christian theology.

Good point @Cody P

I very much concur with @Jpe61 for the origin and definition.
I would add that a synoptic display is more precisely what an aviator or other complex machine operator wants.  It is a visual graphic that conveys the status/summary at a glance without having to actually focus on a read out.
One example would be the picture shown in @JohnK 's answer.  A simple summary page could be done in text, but one would have to read through the text to determine the current status of any given hydraulic pump.  Similar displays are used for fuel tanks.
With the number of instruments and displays in a cockpit, and all the various tasks you have to do, you need all the instruments and displays configured in a way that convey's the most relevant data in the shortest time possible in the most intuitive of ways so the pilot doesn't have to dedicate too much attention to any given display...especially if it's required elsewhere.
As another illustration, there has been some dismay among pilots shifting from analog altimeters to Digital HUD.  The issue is that an analog (needle) display, once you're familiar with it, you can see where the needle is in the scale out of the corner of your eye and you don't have to actually look at it to snap out of a dive at precisely the correct altitude, but with a digital display where the numbers are constantly changing on a steep dive or climb, you actually have to focus on the display to discern the altitude.  That's far less intuitive and can cause a loss of either situational awareness of other events or delay response to other activities requiring concentrated attention. (Such as might be the case while an attack/bomber is attempting to hit a target in hostile airspace --though this example is not so much a problem for commercial or private pilots.)
One final illustration.  If a simple summary or status page in text was sufficient, why wouldn't you simply have a read out in text format of airspeed, altitude, compass heading, etc?  You'd almost be flying a literal desk with Word, PDF, DOS screen, or paper print outs feeding you all your information.  There would be no way for a flight crew to process the information in that medium in a sufficient fashion to be able to effectively fly the plane.
Though it might be easier for a random individual off the street without any training to fill in for any given pilot and be comparably effective.
Synoptic is thus the best and most precise word for the kind of instruments and medium for displaying your situation and status summaries.  Synopsis being the summary data (data or situation at a glance) and optic being the visual medium for that data (presented in ideal visual format).
As for the actual definition: Webster's Fourth ed College Dictionary has

synoptic:1 of or constituting a synopsis; presenting a general view or summary
synopsis:a statement giving a brief, general review or condensation; summary

So synopsis is the summary, synoptic is the presentation of that summary.

Synoptic refers to the active system diagrams that are selectable, usually on one of the middle displays above the center console.  If you want to know what the hydraulic system  is up to, you can select the Hydraulic Synoptic and an active diagram of the hydraulic system appears, showing what's pressurized and what's not, valves open and closed, the pressure values, quantities, etc.  There will be one for most of the major systems, hyper simplified compared to a normal system schematic because only the essential information is presented.

Oddly enough, I was able to find quite a few descriptions for the word, all of them defining synoptic as presenting a summary or a general view of something, and in my opinion, the meaning of the word is best described in Wiktionary by the etymology of the word:

Ancient Greek συνοπτικός (sunoptikós, “seeing the whole together or at a glance”), from σύνοψις (súnopsis, “a general view, synopsis”), from σύν (sún, “with”) + ὄψις (ópsis, “view”)

Synoptic displays are ment to give a simplified, but comprehensive enough view of the current state of a system. You'll find them pretty much everywhere where operators of complex systems need comprehensible information of what is going on in the system, even my car has synoptic displays...
I'd say "system status", "summary page" and "synoptic display" are pretty much the same thing, of course the last being by far the fanciest :)
@John K already described a fine aviation related example of a synoptic display.
Some references:
Wiktionary
Merriam-Webster
Wikipedia
OneLook dictionary Search

Your definition would align with how I've seen 'synoptic' used in scientific data ... for image data, it's something that's been reduced so you can see the whole image on a typical monitor (reduced to 1024x1024 these days) ... for time-series data, it's a plot showing the current value in context so you can easily tell if it's going up/down/etc.